1. Signals

2. Introduction to Signals
What is Signal?
A signal is a software notification to a process of an event.
Why do we need Signals?
In systems we need to enable asynchronous events
Examples of asynchronous events:
message arrives on my machine – mailing agent (user) process should retrieve
Invalid memory access happens – OS should inform scheduler to remove process from the processor
Alarm clock goes on – process which sets the alarm should catch it

3. Basic Signal Concept (Definitions)
Signal is generated when the event that causes the signal occurs.
Signal is delivered when the process takes action based on the signal.
The lifetime of a signal is the interval between its generation and delivery.
Signal that has been generated but not yet delivered is pending.
Process catches signal if it executes signal handler when the signal is delivered.
Alternatively, a process can ignore a signal when it is delivered, that is to take no action.
Process can temporarily prevent signal from being delivered by blocking it.
Signal Mask contains a set of signals currently blocked.

4. How Signals Work Process Signal Generated Signal Mask Signal delivered
Signal Handler
Signal delivered
Signal not blocked
Signal Caught by handler
Return from Signal Handler
Process Resumed
Signal
Mask

5. Examples of POSIX Required Signals
Description
default action
SIGABRT
process abort (can be caught but not ignored)
implementation dependent
SIGALRM
alarm clock
abnormal termination
SIGBUS
Invalid address alignment
Non-existent physical address
SIGCHLD
child terminated, stopped or continued
ignore
SIGILL
invalid hardware instruction
SIGINT
interactive attention signal (usually ctrl-C)
SIGKILL
terminated (cannot be caught or ignored)

6. Signal
Description
default action
SIGSEGV
Invalid memory reference
implementation dependent
SIGSTOP
Execution stopped
stop
SIGTERM
Termination: it can be caught and interpreted (or ignored)
Abnormal termination
SIGTSTP
Terminal stop
SIGTTIN
Background process attempting read
SIGTTOU
Background process attempting write
SIGURG
High bandwidth data available on socket
ignore
SIGUSR1
User-defined signal 1
abnormal termination

7. Generating Signals Signal has a symbolic name starting with SIG
Signal names are defined in signal.h
Users can generate signals (e.g., SIGUSR1)
OS generates signals when certain errors occur (e.g., SIGSEGV – invalid memory reference)
Specific calls generate signals such as alarm (e.g., SIGALARM)

8. Command Line Generates Signals
You can send a signal to a process from the command line using kill
kill -l will list the signals the system understands
kill [-signal] pid will send a signal to a process.
The optional argument may be a name or a number.
The default is SIGTERM.
To unconditionally kill a process, use:
kill -9 pid which is
kill -SIGKILL pid.

9. Command Line Generates Signals
stty –a
CTRL-C is SIGINT (interactive attention signal
CTRL-Z is SIGSTOP (execution stopped – cannot be ignored)
CTRL-Y is SIGCONT (execution continued if stopped)
CTRL-D is SIGQUIT (interactive termination: core dump)

10. Timers Generate SIGALRM Signals
#include <unistd.h>
unsigned alarm (unsigned seconds);
alarm(20) creates SIGALRM to calling process after 20 real time seconds.
Calls are not stacked
alarm(0) cancels alarm

11. Examples: Programming Signals
From a program you can use the kill system call:
#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);
Example 8.4: send SIGUSR1 to process 3423:
if (kill(3423, SIGUSR1) == -1)
perror("Failed to send the SIGUSR1 signal");
Example 8.5: a child kills its parent:
if (kill(getppid(), SIGTERM) == -1)
perror ("Failed to kill parent");

12. Programming Signals if (raise(SIGUSR1) != 0)
Example 8.5: a process sends a signal to itself:
if (raise(SIGUSR1) != 0)
perror("Failed to raise SIGUSR1");
//It is same as kill(getpid(), SIGUSR1);
Example 8.8: kill an infinite loop after 10 seconds:
int main(void) {
alarm(10);
for ( ; ; ) ; }

13. Signal Masks
Process can temporarily prevent signal from being delivered by blocking it.
Signal Mask contains a set of signals currently blocked.
When a process blocks a signal, the OS does not deliver signal until the process unblocks the signal
When a process ignores signal, signal is delivered and the process handles it by throwing it away.

14. Signal Sets #include <signal.h> int sigemptyset(sigset_t *set);
Signal set is of type sigset_t
Signal sets are manipulated by five functions:
#include <signal.h>
int sigemptyset(sigset_t *set);
int sigfillset(sigset_t *set);
int sigaddset(sigset_t *set, int signo);
int sigdelset(sigset_t *set, int signo);
int sigismember(const sigset_t *set, int signo);

15. Signal Sets sigemptyset initializes the set to contain no signals
sigfillset puts all signals in the set
sigaddset adds one signal to the set
sigdelset removes one signal from the set
sigismember tests to see if a signal is in the set

16. Signal Masks SigMask Signal SigInt Bit 2, Signal Sigkill Bit 9,
SigQuit
SigKill …
SigCont
SigAbrt 1 … 1
Signal SigInt Bit 2,
Signal Sigkill Bit 9,
Signal SigChld Bit 20
A SIGSET is a collection of signals:
# is SIGHUP + SIGINT

17. SIGPROCMASK
The function sigprocmask is used to modify the signal mask.
#include <signal.h>
int sigprocmask(int how,
const sigset_t *restrict set,
sigset_t *restrict oset)
If oset is non-null, the previous value of the signal mask is stored in oset.
‘how’ specifies the manner in which the signal mask is to be modified
SIG_BLOCK
The set of blocked signals is the union of the current set and the set argument.
SIG_UNBLOCKED
The signals in set are removed from the current set of blocked signals.
SIG_SETMASK
The set of blocked signals is set to the argument set.

18. Example: Initialize Signal Set:
if ((sigemptyset(&twosigs) == -1) ||
(sigaddset(&twosigs, SIGINT) == -1) ||
(sigaddset(&twosigs, SIGQUIT) == -1))
perror("Failed to set up signal mask");

19. Example: Add SIGINT to Set of Blocked Signals
sigset_t newsigset;
if ((sigemptyset(&newsigset) == -1) ||
(sigaddset(&newsigset, SIGINT) == -1))
perror("Failed to initialize the signal set");
else if (sigprocmask(SIG_BLOCK, &newsigset, NULL) == -1)
perror("Failed to block SIGINT");
If SIGINT is already blocked, the call to sigprocmask has no effect.